Research has shown that people are able to judge sexual orientation from faces with above-chance accuracy, but little is known about how these judgments are formed. Here, we investigated the importance of well-established face processing mechanisms in such judgments: featural processing (e.g., an eye) and configural processing (e.g., spatial distance between eyes). Participants judged sexual orientation from faces presented for 50 milliseconds either upright, which recruits both configural and featural processing, or upside-down, when configural processing is strongly impaired and featural processing remains relatively intact. Although participants judged women’s and men’s sexual orientation with above-chance accuracy for upright faces and for upside-down faces, accuracy for upside-down faces was significantly reduced. The reduced judgment accuracy for upside-down faces indicates that configural face processing significantly contributes to accurate snap judgments of sexual orientation.

This isn’t very surprising, coming as it does as part of a slew of findings–outlined, for instance, in David France’s 2007 New York Magazine article–pointing to fairly deep biological origins for homosexuality, evidenced in variances from heterosexual norms in areas as diverse as fingerprint patterns and brain structure. Subtle differences in facial appearances aren’t out of the realm of the imaginable.

In a recent New York Times opinion peace, Tabak and Zayas described their research and its implications quite nicely.

We conducted experiments in which participants viewed facial photographs of men and women and then categorized each face as gay or straight. The photographs were seen very briefly, for 50 milliseconds, which was long enough for participants to know they’d seen a face, but probably not long enough to feel they knew much more. In addition, the photos were mostly devoid of cultural cues: hairstyles were digitally removed, and no faces had makeup, piercings, eyeglasses or tattoos.

Even when viewing such bare faces so briefly, participants demonstrated an ability to identify sexual orientation: overall, gaydar judgments were about 60 percent accurate.

Since chance guessing would yield 50 percent accuracy, 60 percent might not seem impressive. But the effect is statistically significant — several times above the margin of error. Furthermore, the effect has been highly replicable: we ourselves have consistently discovered such effects in more than a dozen experiments, and our gaydar research was inspired by the work of the social psychologist Nicholas Rule, who has published on the gaydar phenomenon numerous times in the past few years.

We reported two such experiments in PLoS ONE, both of which yielded novel findings. In one experiment, we found above-chance gaydar accuracy even when the faces were presented upside down. Accuracy increased, however, when the faces were presented right side up.

What can we make of this peculiar discovery? It’s widely accepted in cognitive science that when viewing faces right side up, we process them in two different ways: we engage in featural face processing (registering individual facial features like an eye or lip) as well as configural face processing (registering spatial relationships among facial features, like the distance between the eyes or the facial width-to-height ratio). When we view faces upside down, however, we engage primarily in featural face processing; configural face processing is strongly disrupted.

Thus our finding clarifies how people distinguish between gay and straight faces. Research by Professor Rule and his colleagues has implicated certain areas of the face (like the mouth area) in gaydar judgments. Our discovery — that accuracy was substantially greater for right side up faces than for upside-down faces — indicates that configural face processing contributes to gaydar accuracy. Specific facial features will not tell the whole story. Differences in spatial relationships among facial features matter, too.

Consider, for example, facial width-to-height ratio. This is a configural physical feature that differs between men and women (men have a larger ratio) and reflects testosterone release during adolescence in males. Given that stereotypes of gender atypicality — gay men as relatively feminine and gay women as relatively masculine — play a role in how people judge others’ sexual orientation, our finding suggests that cues like facial width-to-height ratio may contribute to gaydar judgments.

Another novel finding: in both experiments, participants were more accurate at judging women’s sexual orientation (64 percent) than at judging men’s (57 percent). Lower gaydar accuracy for men’s faces was explained by a difference in “false alarms”: participants were more likely to incorrectly categorize a straight man as gay than to incorrectly categorize a straight woman as gay.

Why might “false alarm” errors be more common when judging men’s sexual orientation? We speculate that people overzealously interpret whatever facial factors lead us to classify men as gay. That is, it may be that straight men’s faces that are perceived as even slightly effeminate are incorrectly classified as gay, whereas straight women’s faces that are perceived as slightly masculine may still be seen as straight. That would be consistent with how our society applies gender norms to men: very strictly. (Decades of research has established that, at least in our culture, it is considered much more problematic for a boy to play with Barbie dolls than for a girl to play rough-and-tumble sports.)